Solution conformation of a retro-D analogue of tocinamide

The solution conformation of a retro-D analogue of tocinamide H-D-Cys-D-Asn-D-Gln-D-aIle-D-Tyr-NHCH2CH2S was examined using proton magnetic resonance and circular dichroism spectroscopy. The observations support major contributions to the conformational distribution from structures with a type I beta turn in the sequence D-Asp-D-Gln-D-aIle-D-Tyr. This is topologically similar to the beta turn proposed for oxytocin, L-Tyr-L-Ile-L-Gln-L-Asn, but with the polarity of the CONH groups reversed along the chain; the peptide is, however, hormonally inert. In conjuction with nuclear magnetic resonance data, the circular dichroism spectra are interpreted to indicate that the region of the peptide ring near the disulfide occurs in at least two different conformations. One of the side-chain carboxamides, probably that of asparagine, appears to be intramolecularly associated rather than freely exposed to solvent.